Lanthanide-based catalyst systems that comprise a lanthanide compound, an alkylating agent, and a halogen source are known to be useful for producing conjugated diene polymers having high cis-1,4-linkage contents. The resulting cis-1,4-polydienes have a linear backbone, which is believed to provide better tensile properties, higher abrasion resistance, lower hysteresis loss, and better fatigue resistance than those of analogous polymers prepared with other catalyst systems such as titanium-, cobalt-, and nickel-based catalyst systems. Therefore, cis-1,4-polydienes made with lanthanide-based catalyst systems are particularly suitable for use in tire components such as sidewall and tread.
However, due to the linear backbone structure, one disadvantage of cis-1,4-polydienes prepared with lanthanide-based catalyst systems is that the polymers exhibit relatively high cold flow, which can cause problems during storage and transport. The high cold flow also hinders the use of automatic feeding equipment in rubber compound mixing facilities. Another disadvantage of cis-1,4-polydienes prepared with lanthanide-based catalyst systems is that they give relatively high compound Mooney viscosity, which can adversely affect the processability and scorch safety of the rubber compounds. Furthermore, in the art of making tires, it is desirable to employ elastomers that give reduced hysteresis.
Therefore, there is a need to develop a method for producing lanthanide-catalyzed cis-1,4-polydienes that give reduced cold flow, improved processability, and reduced hysteresis.